Aerodynamic caliper gauge

ABSTRACT

A contacting caliper gauge measures the thickness of a moving sheet material. The gauge has two heads, one on either side of the sheet. The sheet contacting pads of the caliper gauge are aerodynamically designed to limit and/or reduce the tendency of boundary layer air travelling with the sheet to raise the sheet-contacting pads off of the sheet surfaces.

BACKGROUND OF THE INVENTION

The present invention is directed to an apparatus and system formeasuring a physical property, such as thickness, of a sheet materialThe present invention is more specifically directed toward a calipergauge for measuring the thickness of a sheet of paper which is in theprocess of being manufactured by a papermaking machine and, therefore,is moving at a high speed through the caliper gauge.

Various types of caliper gauges are known in sensor technology and areused for measuring the thickness of rapidly moving sheet material. Onetype of caliper gauge is called a "contacting caliper gauge". Contactingcaliper gauges typically have two opposing pads which are forced intocontact with opposite sides of the sheet. The distance between the padsis measured and directly related to sheet thickness or "caliper".

Under some situations, however, contacting caliper gauges may sufferfrom certain shortcomings. For example, United States papermanufacturers have formed an association (TAPPI) to promote uniformstandards for the paper industry. Since paper is somewhat compressible,the TAPPI standard for measuring paper sheet caliper requires that themeasurement be based upon results obtained from a contacting calipergauge with a certain specified pressure exerted by the opposing pads onthe paper sheet. Unfortunately, however, the requirement that a calipergauge contact the sheet under pressure poses problems when the caliperof particularly lightweight, thin or weak paper is being measured. Undera fixed pressure, such sheets are prone to being torn by thesheet-contacting pads. This is particularly true when an imperfection orother portion of the sheet which abruptly increases in thickness isdrawn rapidly through the caliper pads.

Another problem with such contacting caliper gauges is caused becausemodern paper mills manufacture paper sheet at high sheet speeds whichcan approach 60 MPH. Such a rapidly moving sheet drags a boundary layerof air along with it such that, near the surface of the sheet, theboundary layer of air is moving at the same speed as the sheet. As thesheet passes between the opposing caliper pads, an "air bearing" effectis created which tends to force the pads away from the surface of thesheet. In this situation, the pads are said to be "flying above" thesheet. Thus, at high paper speeds, the pads of a "contacting" calipergauge may actually fail to contact the sheet. However, as previouslymentioned, conventional caliper gauges determine sheet thickness basedupon the measured distance between opposing pads. Thus, the flyingeffect can induce an erroneous caliper measurement by making the sheetappear thicker than it actually is. The flying problem increases as thesheet speed, and hence the speed of the boundary layer air, increases.In fact, conventional "contacting" caliper gauge designs tend to becomeairborne by up to about 40 microns at higher sheet velocities.Obviously, this is unacceptable when attempting to measure sheet caliperto within 1 micron accuracy under a wide range of sheet velocities.

Previously, caliper designers have given little, if any, considerationto aerodynamics in the design of the caliper pads. For example, certainprior caliper pads have been essentially disk-shaped, with smooth,rounded pad edges to avoid snagging the sheet. These pads tended to flyabove the sheet. To avoid flying,the force on these pads was simplyincreased. However, increasing the force on the contacting caliper padsto counteract the flying effect has not proved to be a satisfactorysolution Increasing the force on opposing pads will tend to counteractthe flying effect resulting from the fast moving boundary layer air, butthis increased force also increases the tendency of the pads to tear thesheet. Accordingly, the present inventors have recognized the need for acontacting caliper gauge which will remain in contact with the sheetsurface (or fly only a very small distance off of the sheet surface)under relatively little external force, while having little or notendency to tear the sheet.

SUMMARY OF THE INVENTION

The present invention relates to a sheet caliper gauge for measuring thethickness of a sheet material while it is moving rapidly in thedirection from the front to the back of the gauge. The gauge comprisesupper and lower pads disposed adjacent to, respectively, upper and lowersurfaces of the sheet to be measured. The pads are held directlyopposite each other, and each pad is forced against the sheet by anelastic extendible member, such as an inflated bellow. Because theelastic members place the pads in forcible contact with the rapidlymoving sheet, it will usually be desirable to make the pads out ofabrasion resistant material.

Each pad has a sheet-contacting surface disposed substantially parallelto the sheet. However, the sheet-contacting surface is preferablybeveled near the front portion of the gauge so that the opposing padstogether form a V-shaped guide for the entrance for the sheet materialbetween the pads. This reduces the probability of tearing the sheet.

As previously explained, the traveling boundary layer of air will tendto create an air bearing which raises each pad off of the sheet surface.However, according to the present invention, this tendency of the padsto fly off of the sheet is reduced or eliminated by providing thesheet-contacting surfaces of such pads with grooves. The grooves are cutin the sheet-contacting surface of the pads lengthwise along thedirection of travel of the sheet, i.e., along the "machine direction".Such grooves vent out the air from the air bearing, and thereby reduceor prevent flying.

Alternatively, or in combination with such grooves, there may be formeda notch in one or both of the opposing pads. This notch is formed in thesheet-contacting surface of a pad and is shaped such that the notch isnarrower, and/or more shallow, toward the front portion of the pad andwidens and/or deepens toward the back of the pad. The notch furtherpreferably has its widest and/or deepest part at the back surface of thepad. Such a tapered notch in the sheet-contacting surface of the padrequires boundary layer air rushing between the pad and the sheet tofill an ever increasing volume as it moves from the front to the rear ofthe pad, thereby creating a partial vacuum in the notch.

The partial vacuum created in the notch tends to force the pad againstthe sheet Such a vacuum is proportional to sheet velocity The liftcaused by the boundary layer air is also proportional to sheet velocity.Thus, by properly sizing and shaping the notch, the partial vacuumcreated in the notch can be made to substantially counteract the speeddependent lifting force of the boundary layer air over a wide range ofsheet speeds. Accordingly, the sheet-contacting pads of the presentinvention remain either in contact with the sheet surface over a broadrange of sheet speeds, or fly only a very small distance off of thesheet over a broad range of sheet speeds. Thus, by measuring thedistance between the opposing pads, the present invention provides anessentially speed-independent sheet caliper measurement which need notbe corrected for flying of the pads above the sheet surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified perspective view of a sheet scanning stationhaving a caliper gauge mounted thereto for measuring sheet thicknessimmediately before the sheet is collected on a reel. Part of the sheetis removed to better illustrate the lower opposing portion of thecaliper gauge.

FIG. 2a is a schematic cross-sectional view of one embodiment of thecaliper gauge of the present invention.

FIG. 2b is an enlarged view of the caliper pads of FIG. 2a.

FIG. 3 illustrates, in perspective, the aerodynamically designed uppercaliper pad of FIGS. 2a and 2b.

FIG. 4 illustrates, in perspective, an alternative design of the caliperpad.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of the sheet thickness or calipergauge 10 of the present invention mounted to a scanner 12 whichtraverses or scans back and forth across the longitudinally moving papersheet 14 being produced by a papermaking machine (not shown). Thescanner 12 is of a now conventional type, such as that described incommonly assigned U.S. Pat. No. 3,621,259 to Mathew G. Boissevain. Thispatent is incorporated herein by reference.

The scanner 12 consists generally of a framework having a pair of spacedupper 16 and lower 18 beams and carriages 20, 22 which move back andforth across the beams 16, 18 in opposed relationship. The uppercarriage 20 carries the upper head 24 of the caliper gauge 10, while thelower carriage 22 carries the lower head 26 of the caliper gauge 10. Thetwo carriages, 20, 22, and thus the two caliper heads, 24, 26, arejuxtaposed to provide a gap therebetween through which the paper sheet14 whose thickness is to be measured freely moves. Although FIG. 1merely shows the caliper gauge 10 used to measure paper thickness, thecarriages 20, 22 would typically also carry additional devices formeasuring various other physical characteristics of the paper sheet 14.

FIG. 2a illustrates, in partial cross-section, the upper and lower heads24, 26 of the present caliper gauge 10. The paper sheet 14 moves rapidlybetween the upper and lower heads 24, 26 in the "machine direction",i.e., the direction shown by the arrow 28. Thus, the paper sheet 14moves from the front of the gauge to the rear of the gauge.

As the caliper gauge 10 is scanned back and forth across the movingpaper sheet 14, signals from the gauge 10 are provided, via signalprocessing circuitry, to the central paper machine process controlcomputer 30. Utilizing these signals, the computer 30 computes a sheetthickness profile. The sheet thickness profile is then used by thecomputer 30 to adjust various sheet processing parameters to create asheet having a desired uniform thickness. FIG. 2a illustrates thatoutput signals 32 from the computer 30 may be used to adjust variousdevices (not shown) on the papermaking machine to achieve the desiredthickness.

The upper head 24 of the caliper gauge 10 includes a sturdy, relativelymassive base 34 mounted to the upper carriage 20. One end of a supportarm 36 is hinged at the front of this base 34. The other end of thesupport arm is hinged at the bellow 38. The bellow 38 connects the otherend of the support arm 36 to the base 34 near the back of the gauge 10.The bellow 38 is disposed substantially perpendicular to the paper sheet14. A sheet-contacting pad 40 is attached to the distal end of thebellow. This pad 40 has a sheet-contacting surface 42 which issubstantially parallel to the sheet 14 near the back of the gauge 10,but which has an inclined portion 44 near the front of the gauge 10 toguide the paper 14 between the pads 40, 46. When pressurized, the bellow38 forces the pad 40 toward contact with the upper surface of the thesheet 14.

The lower head 26 is similar in mechanical construction to the upperhead 24. Like the upper head 24, the lower head 26 includes a secondsturdy, relatively massive base 48, a second sheet-contacting pad 46 anda second bellow 50. Each of these elements are connected insubstantially the same manner as that described above for the upper head24, except, of course, that the lower base 48 is mounted to the lowercarriage 22.

The upper and the lower heads 24, 26 are positioned such that the upperbellow 38 and the lower bellow 50 are in a substantially linear opposingrelationship. Thus, during operation of the gauge 10, the upper pad 40and the lower pad 46 would be disposed in substantial opposingrelationship on opposite sides of the sheet 14.

In general, any extendible means could be used in place of each bellow.However, a bellow is preferred because the electro-magnetic circuit usedto measure the thickness of the sheet material may be placed within thehollow interior of the bellow. One such circuit is fully described incommonly assigned U.S. Pat. Application Ser. No. 3,828,284 to GunnarWennerberg, which is incorporated herein by reference. Briefly, however,the caliper gauge 10 is equipped with an electro-magnetic proximitysensing device for accurately measuring the distance between theopposing pads 40, 46. This device includes an electro-magnet 52 mountedto the upper pad 40 and disposed within the upper bellow 38. This upperpad 40 is preferably formed of a highly abrasion resistant, non-magneticmaterial, such as sapphire. The lower pad 46 is formed of a mageticallysusceptible abrasion resistant material, such as ferrite, preferablycoated with sapphire or diamond. The pads 40, 46 are preferably abrasionresistant to avoid excessive wear caused by the friction between themoving sheet 10 and the sheet-contacting surfaces 42, 54 of the pads 40,46.

The coil 56 surrounding the core of the electro-magnet 52 may beelectrically connected to an oscillator circuit 58 and used as theinductance of that circuit 58. Thus, movement of the magneticallysusceptible ferrite pad 46 toward and away from the coil 56 with sheetthickness changes, changes the inductance of the coil 56 and hence theresonant frequency of the oscillator circuit 58. A frequency counter 60is operatively coupled to the oscillator 58 to determine its resonantfrequency. The counter 60 then sends a signal to the computer 30indicative of this resonant frequency. The computer 30 computes thedistance between the electro-magnet 52 and the ferrite pad 46, and hencesheet thickness, based upon this resonant frequency.

It is particularly preferred that the upper pad 40 be abrasionresistant. This is because, as illustrated in FIG. 2b, theelectro-magnet 52 is recessed within the pad 40. The proximity sensingcircuits are calibrated with the unworn pad. Therefore, if thesheet-contacting surface 42 of the pad 40 is worn down, the poles of theelectro-magnet 52 will move closer to the sheet 14 and produce anerroneous thickness measurement or tear the sheet.

There are a number of advantages to the gauge 10 of the presentinvention which make it well suited to the measurement of the thicknessof thin, weak or otherwise easily damaged material, such as tissuepaper.

The first advantage is that, with the subject gauge 10, the onlyelements which respond to sheet thickness variations are the bellows 38,50, through contraction or elongation, the arms 36, 51, the pads 40, 46and the electro-magnet 52 which move with the bellows 38, 50. Thebellows 38, 50 are much less massive than the upper 34 and lower 48bases. Therefore, the lighter, less massive and therefore more easilymovable bellows 38, 50 reduce the likelihood of tears during themeasurement process when there is a rapid thickness variation in thesheet 14, such as might occur at a sheet imperfection. Also, the arms36, 51 are preferably made of a strong, lightweight material, such asmylar, to further reduce the total mass of the moving parts of the gauge10.

Another advantage of the gauge 10 is the combination of the opposinginclined or beveled portions 44, 62 of the upper and lower pads 40, 46.Together, these inclined portions 44, 62 form a V-shaped guide for theentrance of the sheet 14 material between the pads 40, 46. As can bestbe seen in FIGS. 3-4, discussed more fully below, the pads arepreferably rectangular. Therefore, the V-shaped inclined rectangularfront portions 44, 62 distribute force evenly over the entire width ofthe pads 40, 46. This even distribution of pressure over the entirewidth of the rectangular pads decreases localized forces between thesheet 14 and the pads 40, 46, thereby still further reducing theprobability of tearing the sheet 14.

Further, as described in greater detail below, the aerodynamic action ofthe pad minimizes and/or completely counteracts the lifting action ofthe air bearing effect previously discussed.

FIG. 3 illustrates one aerodynamic pad design 40. The pad 40 may be 0.75inch wide measured along the "cross-direction" perpendicular to thedirection of sheet movement, by 1.00 inch along the machine direction,by 0.03 inch thick. The front of the pad 40 preferably has a 0.12 inchlong 2° inclined surface 44 measured relative to the plane of the sheet14. The central and back portions of the sheet-contacting pad 40 surfacehave a "vacuum notch" 68 formed therein.

As previously discussed, a boundary layer of air travels with the sheet14 as the sheet 14 passes between the two opposing pads 40, 46. Thisboundary layer of air will tend to lift the pads 40, 46 off the surfacesof the sheet, thereby producing a false sheet thickness measurement.With the aerodynamic pad design of FIG. 3, as the sheet speed isincreased, the boundary layer of air first forces itself between the pad40 and the sheet 14 near the front portion of the pad 44, therebytending to raise the pad 40 off of the surface of the sheet 14. However,as the boundary layer reaches the vacuum notch 68 and continues totravel toward the back of the gauge 10, it is forced to fill an everincreasing volume (see e.g., FIG. 2b), thereby creating a partial vacuumwithin the notch 68 between the pad 40 and the sheet 14. This partialvacuum pulls the pad 40 back toward the sheet 14, thus counteracting theair bearing effect. Moreover, as the speed of the sheet 14, and hencethe air bearing effect, increases, the partial vacuum formed in thevacuum notch 68 also increases, thereby tending to cancel theincreasingly strong air bearing effect. The net result is that, with thepad design of FIG. 3, the tendency of the pad 40 to fly off of the sheetwill be reduced or eliminated over a wide range of paper speeds. In theillustrated embodiment of FIG. 3, the vacuum notch 68 is 0.80 inch longand increases to 0.40 inch wide and 0.018 inch deep at the rear surfaceof the pad 40. However, the dimensions of the notch 68 and pad 40 may beadjusted for use in different manufacturing situations to provide thedesired pressure between opposing pads. All else being equal, a widerand/or deeper vacuum notch will provide a greater partial vacuum than anarrower, more shallow vacuum notch. In fact, the "notch" may actuallyextend at all points across the entire width of the pad so that theentire rear portion of the sheet-contacting pad surface is inclined awayfrom the sheet surface. Whatever dimensions are chosen, all pad surfacesshould be smooth to avoid build-up of paper dust which could cause anerroneous measurement.

We have found that an appropriate method of creating the vacuum notch 68is by grinding the pad 40 with a grinding wheel having an 1.5 inchdiameter. The grinding wheel is oriented with its axis of rotationlocated parallel t the machine-direction axis of the pad and about onedegree from a plane parallel to the face of the pad 40. Then thegrinding wheel is moved from one end of the pad to the other so that thenotch 68 is formed having the shape of a section of a cylinder.

Pads of identical design may, if desired, be used on both the upper 24and lower 26 heads of the caliper gauge 10. However, in certainsituations, it may only be necessary to use the pad design of FIG. 3 onone of the two heads. The opposing head may simply be formed with a padhaving a flat sheet-contacting surface, as shown in FIGS. 2a and 2b.These figures illustrate a caliper gauge 10 wherein only the upper head24 utilizes a pad 40 having a vacuum notch 68. Despite the fact that avacuum notch 68 is formed in only one of the two opposing pads, thepartial vacuum created on one side of the sheet 14 by the vacuum notch68 will nevertheless also cause a low pressure region on the oppositeside of the sheet. This low pressure region on the opposite side of thesheet 14 also tends to force the opposing flat-surfaced pad 46 towardthe sheet. Thus, although only a single pad need be formed with anaerodynamic vacuum notch, the resulting partial vacuum will force bothpads 40, 46 toward contact with the sheet 14.

The low pressure region between the sheet 14 and the flat-surfaced pad46 may be caused by one or both of the following effects. First, if thesheet is porous, like many paper sheets, the boundary layer air betweenthe sheet 14 and the flat-surfaced pad 46 will be sucked through theporous sheet 14 toward the vacuum notch 68, thereby causing a partialvacuum in the space between the sheet 14 and the lower pad 46.Alternatively, or in addition, even if the sheet 14 is not porous, thesheet 14 will tend to be sucked into, and therefore conform to the shapeof the vacuum notch 68. Thus, the sheet itself will obtain a aerodynamicshape which will create a partial vacuum adjacent the flat-surfacedopposing lower pad 46. In either event, both pads 40, 46 will be drawntoward the sheet 14 by an aerodynamically created partial vacuum.

FIG. 4 illustrates, in perspective, an alternative design for thesheet-contacting pad 70. According to this design, grooves 72 are cut inthe pad 70 lengthwise along the machine direction. These grooves 72extend from the front to the back of the pad 70. Five evenly spaced 72grooves may be formed in the pad 70, each groove 72 being 0.005 inchdeep and 0.040 inch wide. These grooves 72 are spaced at 1/8 inchintervals. The overall length, width and thickness of the pad 70 is thesame as that for the above-described embodiment of FIG. 3.

The grooves 72 cut in the pad 70 of FIG. 4 provide channels for theboundary layer air to flow between the pad 70 and the sheet 14, whilestill permitting portions of the pad 70 to remain in contact with thesheet. This release, or venting, of boundary layer air through themachine directionally oriented grooves 72 improves contact with thesheet, and therefore caliper accuracy. As in the embodiment o FIGS. 2-3,one or both of the opposing pads may include the grooves 72. Also, as inthe embodiment of FIG. 2-3, one pad will be made of a magneticallysusceptible material, while the opposing pad will have an electro-magnet74 for determining the distance between the two pads. Whichever paddesign is utilized, the magnetically susceptible pad should besufficiently broad in lateral extent that slight lateral misalignmentsbetween the upper and lower heads 24, 26 will not induce a falsely largecaliper measurement. In operation, the sheet 14 is threaded between theopposing caliper heads 24, 26 and the central paper mill process controlcomputer 30 instructs the scanning station 12 to begin scanning thecaliper gauge 10 back and forth along the cross-direction of the sheet14. The bellows 38, 50 are pressurized to place the pads in forcibleopposing contact with the sheet 14. A pressure of about 2-4 inches ofwater (gauge) in 1 inch diameter bellows will provide sufficientpressure to maintain the pads of FIG. 3 in contact or very closeproximity (less than about 2 microns) to the sheet over a relativelywide range of sheet speeds. A pressure of about 5-7 inches of water(gauge) in the same size bellows will suffice to maintain the pads ofFIG. 4 in contact or in very close proximity with the sheet over arelatively wide range of sheet speeds. Of course, higher bellowspressures may be used with stronger, less easily damaged sheets. Sheetcaliper measurements are performed by the proximity sensing electronics,as discussed above.

As the sheet speed increases, a rapidly moving boundary layer of airwill form near the opposing sheet surfaces and attempt to lift the padsaway from contact with the sheet. The aerodynamically designed pads ofthe present invention will counter this effect. However, in addition,air pressure may also build up on the inner front surfaces of the upper36 and lower 51 arms. Such pressure would also tend to raise the padsoff of the sheet 14. However, the arms 36, 51 are designed with ventholes 76 (FIG. 3) which allow the air to flow through the arms, 36, 51,thus relieving the pressure and, again, minimizing the tendency of thepads to fly off of the sheet surfaces These vent holes 76 also reducethe total weight of the moving parts of the caliper gauge 10 and,therefore, make it more responsive to rapid changes in sheet thicknesswith reduced risk of tearing the sheet.

Two preferred embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention Forexample, either of the pads illustrated in FIGS. 3 and 4 may be usedwith a single caliper head, or on both opposing caliper heads. A padhaving, in combination, both one or more machine directionally orientedgrooves and one or more vacuum notchs would be well within the scope ofthe present invention. Also, grooves and vacuum notches of varying sizesand shapes are also within the scope of the invention. Moreover, thepresent invention is also not limited to use with paper sheet nor to usewith electro-magnetic proximity sensing devices. Other sheet materialand other proximity sensing devices may be used. Furthermore, physicalcharacteristics of the sheet other than thickness may be measured withthe disclosed invention. Thus, the present invention is not limited tothe preferred embodiments described herein, but may be altered in avariety of ways which will be apparent to persons skilled in the art.

We claim:
 1. An aerodynamic sheet contact pad, the pad having front andback portions and a first side, the first side including a first surfacefor contact with a moving sheet and at least one indentation in thefirst side extending from the back end of the pad to a location alongthe first surface spaced from the back end of the pad.
 2. The sheetcontact pad of claim 1, wherein the first side of the pad furtherincludes a beveled rectangular surface at the front end of the padextending across the width of the pad.
 3. The sheet contact pad of claim1, wherein the indentation increases in size toward the back end of thepad.
 4. The sheet contact pad of claim 2, wherein the indentationincreases in size toward the back end of the pad.
 5. The sheet contactpad of claim 1, wherein the indentation is a groove extending from theback end of the pad toward the front of the pad.
 6. The sheet contactpad of claim 2, wherein the indentation is a groove extending from theback end of the pad to the beveled surface.
 7. An aerodynamic sheetcontact pad, the pad having front and back portions and a first side,the first side including a first surface for contact with a moving sheetand a beveled surface at the back end of the pad inclined away from thesheet contact surface, the beveled surface extending across the entirewidth of the pad.
 8. A caliper gauge for measuring a physical propertyof a sheet material, the gauge comprising:a first base; an extendiblemember having one end connected to the base; and a first pad mounted tothe end of the extendible member opposite the first base, the first padhaving a first side opposite the base and a front and back end, whereinthe first side includes a first surface having an indentation extendingfrom the back end of the first pad to a location along the first surfacespaced from the back end of the first pad.
 9. The caliper gauge of claim8, wherein the first pad further includes a beveled rectangular surfaceat the front end of the first pad extending across the entire width ofthe first pad.
 10. The caliper gauge of claim 8, wherein the indentationincreases in size toward the back end of the first pad.
 11. The calipergauge of claim 9, wherein the indentation increases in size toward theback end of the first pad.
 12. The caliper gauge of claim 8, wherein theindentation is a groove extending from the back end of the first padtoward the front of the first pad.
 13. The caliper gauge of claim 9,wherein the indentation is a groove extending from the back end of thefirst pad to the beveled surface.
 14. The caliper gauge of claim 8,further comprising a first arm, one end of the arm being connected tothe base and the other end of the arm being connected to the first pad.15. The caliper gauge of claim 8, further comprising a second base;asecond extendible member having one end connected to the second base;and a second pad mounted to the end of the second extendible memberopposite the second base, the second pad having a second side oppositethe second base and a front and back end, wherein the second sideincludes a second surface having an indentation extending from the backend of the second pad to a location along the second surface spaced fromthe back end of the second pad.
 16. The caliper gauge of claim 15,further comprising means for determining the distance between the firstand second pads.
 17. The caliper gauge of claim 16, wherein the meansfor determining the distance between the first and second pads includesan electro-magnetic proximity sensing device.
 18. A sheet measuringsystem, comprising:a first sheet-contacting pad, the first pad having afirst side and a front and back end, wherein the first side includes afirst sheet-contacting surface having an indentation extending from theback end of the first pad to a location along the first surface spacedfrom the back end of the first pad; a sheet moving from the front to theback end of the first pad and having first and second sheet surfaces onopposite sides of the sheet, the moving sheet moving so that the firstsheet surface is adjacent to the first sheet-contacting surface; and asecond sheet-contacting pad, the second pad having a second side and afront and back end, wherein the second side includes a secondsheet-contacting surface having an indentation extending from the backend of the second pad to a location along the second surface spaced fromthe back end of the second pad, the second sheet-contacting surfacebeing disposed adjacent to the second surface of the moving sheet. 19.The sheet measuring system of claim 18, further comprising:meansoperatively coupled to at least one of the pads for measuring thedistance between the first and second pads; and means for biasing thefirst and second pads toward the sheet.